1. Field of the Invention
The present invention relates to processes for depositing thin semiconductor films, and particularly to a process for thermally depositing a thin semiconductor film excellent in photoelectric characteristics from a gaseous starting material.
2. Description of the Prior Art
As one of conventional processes for thermally depositing a thin silicon film from a gasous starting material (thermal CVD method), there is known a technology of depositing a thin amorphous, singlecrystalline or polycrystalline film from a silane gas. The lowest temperature of a substrate permitting of film formation is 500.degree. C. when monosilane (SiH.sub.4) is used as the silane gas, 425.degree. C. when disilane (Si.sub.2 H.sub.6) is used, and 400.degree. C. when trisilane (Si.sub.3 H.sub.8) is used. At such a temperature, however, the deposition rate is usually about 1,000 .ANG. per hour. Thus, deposition in practice must be effected at a temperature at least 50.degree. C. higher than the above-mentioned temperature. Such high-temperature deposition, or film formation, inevitably restricts the kind of substrate, in particular, a glass substrate is unusable. In addition, the photoconductivity of a film deposited by such a thermal CVD method is lower by at least two figures than a film deposited by a plasma CVD method.
On the other hand, a thermal CVD amorphous silicon film is excellent in other characteristics such as a resistance to photo-induced degradation by light. Under such circumstances, there has been a demand for a process for depositing a thin semiconductor film at a high deposition rate while making much of the merits of the thermal CVD method as well as a technology of improving the photoelectric characteristics of a film of the kind as described above. This problem has been the same also with a thin germanium film and a thin carbon-containing silicon film.
It is known that the photoelectric characteristics of a thin film can be improved by bonding hydrogen atoms to defects such as dangling bonds of silicon, germanium, carbon or like atoms. When a thin film formed by the thermal CVD method was taken out of film forming equipment into the ambient air and then exposed to a hydrogen plasma, however, no substantial improvement was recognized in the photoconductivity of the film.